With the UT-Bureau Of Economic Geology’s Scale Calculator, enter a map scale and it calculates what a measurement on the map represents in reality, or what a unit distance translates to at that map scale:
There are also links at the top of the page to other calculators for area/distance conversion, and decimal degree / deg-min-sec degree conversions (both ways).
The OSU Scale Calculator is a bit different – enter a map measurement distance for a unit distance, and get back the map scale number:
There’s also another calculator for basic distance unit conversions, plus also conversions from degrees of slope to % grade and back:
Via Kelso’s Corner (which also has some useful tables and info related to map scale),